As a discharge lamp lighting device that lights a discharge lamp of which representative is a fluorescent lamp, a general one is an electronic ballast that lights the discharge lamp at a high frequency by using an inverter circuit that converts a commercial alternating current voltage into a direct current voltage and converts this direct current voltage into a high frequency voltage. As such an electronic ballast, there is one that has a dimming function capable of changing brightness of the discharge lamp by increasing or decreasing an amount of supplied power to the discharge lamp in response to fluctuations of a dimming signal inputted to the electronic ballast concerned from the outside. The electronic ballast added with this dimming function is generally used for saving energy, and for a stage direction. The discharge lamp lighting device added with the dimming function, which is as described above, is required to have stable dimming performance that does not allow occurrences of unstable phenomena such as variations and flicker of a light output, for example, up to a low-luminous-flux dimming range such as 10% or less of a rating light output.
As the discharge lamp lighting device, there is known one that, in order to respond to such a requirement, detects a lighting state of the discharge lamp, and performs feedback control so that the output of the discharge lamp can become a predetermined output in response to the inputted dimming signal. For this feedback control, there are generally used: a method in which a lamp current flowing through the discharge lamp is detected, and the feedback control is performed so that a value of the detected lamp current can become a predetermined current value corresponding to the dimming signal; and a method in which the lamp power supplied to the discharge lamp is detected, and the feedback control is performed so that a value of the detected power can become a predetermined power value corresponding to the dimming signal.
FIG. 11 is a circuit diagram showing a representative configuration of a conventional discharge lamp lighting device. In general, a direct current voltage source E1 can be easily composed of a configuration of rectifying a commercial power supply by a full-wave rectification circuit and thereafter smoothing the rectified commercial power supply by a capacitor, or of an AC/DC conversion circuit such as a step-up chopper circuit. The direct current voltage source E1 generates a direct current voltage Vdc.
In this discharge lamp lighting device, between a positive electrode and negative electrode of the direct current voltage source E1, a series circuit having a high-side switching element Q1, a low-side switching element Q2 and a resistor R1 is connected. This discharge lamp lighting device composes a half-bridge inverter circuit INV (alternating current output circuit) that converts the direct current voltage Vdc into a high-frequency voltage by alternately switching the switching elements Q1 and Q2 at a high frequency.
Moreover, in the discharge lamp lighting device, both ends of a series circuit of the switching element Q2 and the resistor R1 compose output ends of the inverter circuit INV. Between these output ends, a series circuit of an inductor L1 and a capacitor C1 is connected, further, between both ends of the capacitor C1, a series circuit of a capacitor C2 and a fluorescent lamp (discharge lamp) FL is connected, and a resonance circuit is composed of the inductor L1 and the capacitors C1 and C2. The discharge lamp lighting device applies a substantially sinusoidal high-frequency voltage to the fluorescent lamp FL, and thereby lights the fluorescent lamp FL at a high frequency.
Between the output ends of the inverter circuit INV, a series circuit of a primary winding of a transformer T1 and a capacitor C3 for cutting the direct current is further connected. To both ends a and b and both ends c and d of two sets of secondary windings of the transformer T1, both ends of respective filaments of the fluorescent lamp FL are connected through capacitors C4 and C5, respectively. Preheating currents for appropriately heating the respective filaments of the fluorescent lamp FL are supplied thereto from the transformer T1.
Moreover, the resistor R1 connected in series to the switching element Q2 detects currents flowing through the switching element Q2, and equivalently detects an average of powers outputted from the inverter circuit INV based on an average value of the detected currents. Such average power of the inverter circuit INV, which is detected by the resistor R1, is inputted to an inverting input terminal of an operational amplifier OP1 through a resistor R2. To a non-inverting input terminal of the operational amplifier OP1, a dimming command value voltage Va10 outputted from a dimming command value control circuit 5 in response to a level of a dimming signal Vs1 inputted thereto from the outside is inputted. In the operational amplifier OP1, a capacitor C6 is connected between the inverting input terminal thereof and an output terminal thereof, and the operational amplifier OP1 compares these two inputs with each other, and changes an output voltage representing a difference therebetween.
To the output terminal of the operational amplifier OP1, an inverter control circuit 6 is connected. The inverter control circuit 6 changes switching frequencies of the switching elements Q1 and Q2 in response to the output voltage of the operational amplifier OP1. In such a way, the inverter control circuit 6 controls output power of the inverter circuit INV, and performs feedback control so that a both-end voltage of the resistor R1, which is equivalent to the output power of the inverter circuit INV, can become substantially the same as the dimming command value voltage Va10. Moreover, the dimming command value control circuit 5 appropriately changes the dimming command value voltage Va10 in response to the level of the dimming signal Vs1, thereby adjust the output power of the inverter circuit INV in response to the dimming signal Vs1, and dims the fluorescent lamp FL.
Moreover, a series circuit of resistors R12 and R13 connected between one end of the fluorescent lamp FL and a low-voltage-side output (ground level) of the direct current voltage source E1, and a capacitor C12 connected in parallel to the resistor R13, compose a direct current voltage detection circuit 2. This direct current voltage detection circuit 2 performs voltage division for a both-end voltage of the fluorescent lamp FL by the resistors R12 and R13, and smoothes the both-end voltage concerned by the capacitor C12, thereby detects a direct current voltage component of the high-frequency voltage generated on both ends of the fluorescent lamp FL. Moreover, an increase of the direct current voltage component owing to a rectification function (asymmetry of lamp current owing to half-wave discharge) of the fluorescent lamp FL at the time of an end of a lifetime of the fluorescent lamp FL is detected by a comparator circuit 20. In the case where the direct current voltage component reaches a predetermined value, an oscillation stop signal Vr10 is outputted from the comparator circuit 20 to the inverter control circuit 6, and the inverter control circuit 6 stops switching operations of the switching elements Q1 and Q2, and thereby prevents an occurrence of excessive stresses to the fluorescent lamp FL and such circuit parts at the time of the end of the lifetime.
Moreover, there is also a discharge lamp lighting device that makes feedback of the lighting state of the discharge lamp and gives a direct current bias to the discharge lamp in order to prevent fading of the discharge lamp at the time of low-luminous-flux dimming thereof (for example, Japanese Patent Laid-Open Publication No. 2002-75681).
As described above, in the general discharge lamp lighting device that detects the lamp current flowing through the discharge lamp and the lamp power supplied to the discharge lamp and performs the feedback control for the power supplied to the discharge lamp, the lamp current and the lamp power are lowered as a dimming ratio is lowered (light output is lowered). Therefore, for example, in such a low-luminous-flux dimming range where the dimming ratio is 10% or less of the rating light output, detection values of the lamp current and the lamp power become micro values, and accuracy of the feedback control is deteriorated. Therefore, in particular, in the case where the ambient temperature of the discharge lamp is low, there have been problems that it becomes difficult to maintain the lighting state of the discharge lamp, whereby the fading occurs, as well as that the light output of the discharge lamp is lowered, whereby the flicker is prone to occur.
The present invention has been made in consideration for the above-described circumstances. It is an object of the present invention to provide a discharge lamp lighting device capable of enhancing stability of the discharge lamp at the time of dimming lighting even in the case where the light output is lowered, and to provide an illuminating device.